1. Field of the Invention
The invention relates to a method of producing a nanocomposite thermoelectric conversion material in which nano-sized phonon-scattering particles are dispersed in a matrix made of a thermoelectric conversion material, and a nanocomposite thermoelectric conversion material produced using the method.
2. Description of the Related Art
A thermoelectric conversion material is an energy material that directly converts thermal energy to electric energy, based on two basic thermoelectric effects, that is, the Seebeck effect and the Peltier effect.
A thermoelectric generation device, which uses the thermoelectric conversion material, has many advantages as compared to conventional power generation technologies. For example, the thermoelectric generation device has a simple structure, and is robust and highly durable. The thermoelectric generation device does not have a movable member. The micro-sized thermoelectric generation device is easily produced. The thermoelectric generation device does not require maintenance. The thermoelectric generation device is highly reliable, has a long lifespan, does not cause noise, and does not cause contamination. The thermoelectric generation device uses low-temperature waste heat.
A thermoelectric cooling device, which uses the thermoelectric conversion material, also has advantages as compared to conventional compression cooling technologies. For example, the thermoelectric cooling device does not require chlorofluorocarbon, and does not cause contamination. The small-sized thermoelectric cooling device is easily produced. The thermoelectric cooling device does not have a movable member, and does not cause noise.
Therefore, particularly because energy-related issues and environment-related issues have recently become more serious, it is expected that the thermoelectric conversion material will be put to practical use in fields of aerospace, national defense, construction, geological observation, weather observation, medical care, hygiene, microelectronics, and the like. Also, it is expected that the thermoelectric conversion material will be used for various purposes, for example, for the purpose of using waste heat in petrochemical industry, metallurgy, and electric power industry.
A power factor P=S2σ, and a nondimensional performance index ZT=(S2σ/κ) T are used as indices for evaluating the performance of the thermoelectric conversion material. In this case, S represents a Seebeck coefficient, σ represents an electric conductivity, κ represents a thermal conductivity, and T represents an absolute temperature. That is, in order to obtain a good thermoelectric characteristic, the Seebeck coefficient S and the electric conductivity σ need to be high, and the thermal conductivity κ needs to be low.
To scatter phonons, which conduct heat, is effective for decreasing the thermal conductivity κ. Thus, a composite thermoelectric conversion material, in which particles used for scattering the phonons (hereinafter, referred to as “phonon-scattering particles”) are dispersed in a matrix made of a thermoelectric conversion material, has been proposed.
Japanese Patent Application Publication No. 2000-164940 (JP-A-2000-164940) describes a technology in which particles of Ag2Te (that is a thermoelectric conversion material) are dispersed in a matrix made of a thermoelectric conversion material AgBiTe2. The phonons are scattered by the dispersed particles, and thus, the thermal conductivity κ is decreased. In a production method thereof, a mixture of a matrix material and the dispersed particles is molten; the dispersed particles are uniformly precipitated by maintaining the temperature of the mixture at a temperature equal to or higher than the melting point of the matrix material, and equal to or lower than the melting point of the dispersed particles; and then, the matrix is precipitated by cooling the mixture to a temperature equal to or lower than the melting point of the matrix. Accordingly, the dispersed particles are precipitated before the matrix is precipitated. Therefore, the dispersed particles are likely to agglutinate, and thus, the size of the dispersed particles is likely to be increased. As a result, the dispersibility of the dispersed particles is decreased, the effect of scattering the phonons is decreased, and the effect of decreasing the thermal conductivity is decreased.
Japanese Patent Application Publication No. 2000-261047 (JP-A-2000-261047) describes a technology in which ceramic particles are dispersed in a matrix made of a thermoelectric conversion material CoSb3. In this method, however, even the smallest dispersed ceramic particles are sub-micron sized particles. Therefore, the phonons are not highly scattered. Even if the nano-sized ceramic particles were used, the nano-sized ceramic particles would be likely to agglutinate, and as a result, the dispersed particles would not be nano-sized particles.
Japanese Patent Application Publication No. 2003-73705 (JP-A-2003-73705) describes a technology in which a reducing agent is dropped into a solution containing a metal with a high reduction potential and a metal with a low reduction potential, and the metal with the high reduction potential is precipitated first, and then, the metal with the low reduction potential is precipitated. However, the publication No. 2003-73705 merely describes the fact that metals are reduced and precipitated in order of decreasing the reduction potential. The publication No. 2003-73705 does not describe a technology in which phonon-scattering particles are dispersed in a matrix made of a thermoelectric conversion material.
Japanese Patent Application Publication No. 2008-305919 (JP-A-2008-305919) describes a technology in which a solution containing salts of elements constituting a thermoelectric conversion material and a salt of a metal or an alloy is mixed with a solution containing a reducing agent. The amount of the salt of the metal or the alloy contained in the solution is excessive with respect to the amount of the metal or the alloy to be contained in the thermoelectric conversion material. Particles including the elements constituting the thermoelectric conversion material and particles including the metal or the alloy are precipitated, and heat treatment is performed. Thus, the particles including the metal or the alloy are dispersed in the matrix made of the thermoelectric conversion material. Then, sintering is performed, and thus, the particles including the metal or the alloy are integrated with the matrix. In this method, however, the particles including the elements constituting the thermoelectric conversion material of the matrix, and the phonon-scattering particles are precipitated in a mixed state. Therefore, it is not possible to prevent the agglutination of the precipitated phonon-scattering particles, and the increase in the size of the phonon-scattering particles.